(a) Field of the Invention
The present invention is related generally to a superheterodyne radio receiver, and more particularly, it pertains to an improvement of a superheterodyne radio receiver for detecting and preventing the occurrence of an interference by an interfering station signal at a frequency close to that of a desired station signal to be received by the receiver.
(b) Description of the Prior Art
In case the receiving antenna of a radio receiver catches, in addition to a desired station signal to be received by the receiver, interfering another stronger station signal, i.e. adjacent-channel station signal, at a frequency close to that of the desired station signal, the receiver finds it difficult to receive the desired station signal in a good condition. As the counter-measures which have been taken in the past to cope with the occurrence of an interference by such adjacent-channel station signal, the following means are provided in radio receivers. They include: (a) the pass-band width of band-pass filters in either the RF or IF amplifier stage is arranged narrow; (b) provision of a wider dynamic range of the RF amplifier stage, the mixer stage and the IF amplifier stage; and (c) application of automatic gain control techniques. However, in each of these efforts in the prior art, there indeed has been a limit in the ability of excluding such interference by an adjacent-channel station signal, owing to the restrictions encountered in the arrangement of circuitry of the radio receiver.